1. Technical Field
The present invention relates generally to a process wherein nutrients and/or microbial organisms are injected into underground formations for microbial processes, and in particular to a microbial enhanced oil recovery (“MEOR”) process for heavy oil accumulations.
2. Description of the Related Art
Production of heavy oil from unconsolidated reservoirs, like the ones around Lloydminster which straddles the provinces of Alberta and Saskatchewan in Canada, continued for decades while trying to prevent sand production with screens or gravel packs. The oil and gas industry realized in the 1980s that if sand production was encouraged, that oil production also increased. A non-thermal process was developed known as Cold Heavy Oil Production with Sand (CHOPS) in which sand and oil were produced simultaneously under primary conditions. Progressive cavity pumps were typically deployed in a CHOPS process and allowed sand production and higher levels of oil production to be reached over prior approaches.
As a result of producing sand from these reservoirs, pathways of extremely high permeability are generated in oil producing formations. These high permeability pathways are known as “wormholes”. As the sand production is continued, wormholes grow larger and extend deeper into the reservoir. The presence of wormholes has been proposed in light of the observations in these oil fields and from investigations through laboratory experiments (Tremblay, B., Sedgwick, G. and Vu, D., “CT Imaging of Wormhole Growth Under Solution-Gas Drive”, SPE Reservoir Evaluation & Engineering, Vol. 2, No. 1, February 1999, 35-47). Numerous tracer surveys were conducted where rapid communication was observed between wells confirming the existence of wormhole structures. Pressure-buildup analyses conducted throughout the Lloydminster area showed in-situ permeability values on the order of tens of darcies, which was much higher than anything measured in the laboratory (Smith, G. E., “Fluid Flow and Sand Production in Heavy Oil Reservoirs Under Solution Gas Drive”, SPE Production Engineering, May 1988, 169-180). Such values are theorized to be due to the flow through the high permeability channels or wormholes. The test results also indicated that wells have very large “wellbore storage,” even for the wells that were shut in downhole. Furthermore, laboratory experiments showed that a stable wormhole can develop in unconsolidated heavy oil sands and that the wormholes most likely develop in a higher porosity region with lower cohesive strength (Tremblay et al., supra). Tracer tests conducted by injecting in one well and detecting the arrival times in the surrounding wells sometimes indicated travel times in the order of hours, lent further credence to the existence of wormholes in the reservoir. It is thought that near the wellbore a denser network/dilated region is formed and a few of these wormholes grow up to 50 to 200 m in length in time (Smith, supra). FIG. 1 (PRIOR ART) shows a schematic of aerial view of a CHOPS well 1 with associated wormhole network 2.
Solution gas drive in these reservoirs involves simultaneous mixture flow of gas as very tiny bubbles entrained in viscous heavy oil, also called foamy oil flow. Foamy oil flow is a result of nonequilibrium thermodynamics. Therefore, two significant mechanisms which are theorized to affect the flow of heavy oil and its recovery in these reservoirs are the foamy oil flow and wormhole formation (Sawatzky, R., Lillico, D. A., London, M., Tremblay, B. R., and Coates, R. M., “Tracking Cold Production Footprints”; paper 2002-086, presented at the Canadian International Petroleum Conference, Calgary, AB, Jun. 11-13, 2002). The primary CHOPS production wells come to the end of their lives either due to pressure depletion or due to excessive water influx. In general, the primary recovery in heavy oil reservoirs ranges between 3 to 10% with average of around 5% recovery (Smith, supra). Although a few enhanced oil recovery (“EOR”) techniques have been tried, currently there are no widely applicable commercial EOR techniques to increase the recovery of cold heavy oil beyond the primary levels.
Water flooding of heavy oil is inefficient. Water will bypass the oil and breakthrough at the producers early in the life of the flood because of viscous instabilities resulting from the adverse mobility contrast between water and heavy oil. Many of these reservoirs are relatively small or thin, and possibly have existing primary production wormholes. Consequently, these reservoirs are not prime candidates for expensive thermal or miscible hydrocarbon solvent EOR technologies. Wormholes negatively affect water flood performance as well (Bryan, J., Mai, A., and Kantzas, A, “Processes Responsible for Heavy Oil recovery by Alkali/Surfactant Flooding”, JPT, January 2009, 52-54). Considerable water is produced sometimes in these reservoirs during primary operations. As long as water production is low, quite high sand cuts can be tolerated by the production system. If wormholes reach a water source, water will short circuit through them and the well will be suspended. Many sudden failures in injection schemes (firefloods, water floods, and steam floods) and in drilling and workover operations are also blamed on wormholes.